Chlorine dioxide is an unusual, nominally gaseous, chemical compound (b.p.=11.degree. C. @ 1 atm), which exhibits desirable reactive properties for industrial applications involving several different important raw materials, such as the purification of water and other liquids and the bleaching of fibrous solids, especially wood pulps. However, due to its inherent chemical instability, chlorine dioxide cannot be stored or shipped satisfactorily for meeting normal commercial needs. Instead, it is generally produced as needed at or adjacent to the particular treatment facility in question.
In most such industrial installations, chlorine dioxide is generated by chemically reducing a metal chlorate in a strong acidic aqueous solution. Among the most prominent of the reducing agents currently employed for this purpose are chloride ion (Cl.sup.-), sulfur dioxide and methanol. Similarly, sodium chlorate is the most common chlorate raw material, while sulfuric and/or hydrochloric are the major acids of general choice. Regardless of the specific combination of reactants chosen, the chlorine dioxide producing reaction is always conducted with the utmost care to avoid the formation of potentially explosive mixtures thereof. Thus, not only are moderate reaction temperatures (typically about 20.degree. to 80.degree. C.) carefully maintained, but also the partial pressure of chlorine dioxide in the gas phase evolved is rigorously limited, e.g. by operating the generator at a substantial vacuum (thus reducing total pressure while also increasing the proportion of water vapor evolved) and/or by the use of an inert gas sweep such as air to reduce the ClO.sub.2 concentration in the gaseous mixture evolved. The general reliance upon such safety measures in the achievement of practical large scale production of ClO.sub.2 is documented in many prior art disclosures, of which U.S. Pat. Nos. 3,446,584, 3,864,456 and 3,933,987 are representative.
Unfortunately, the highly diluted, low pressure gas mixtures of ClO.sub.2 obtained by observing these customary safety precautions represent such an extended, bulky and voluminous form of ClO.sub.2 that further extensive handling or direct use thereof is unduly difficult and generally uneconomic. As a practical matter, therefore, in most state of the art procedures for continuous, on-site production of chlorine dioxide, the diluted, low pressure gas mixtures initially generated are promptly contacted with a stream of chilled water to effect absorption and dissolution therein of most of the chlorine dioxide. (See, for example, U.S. Pat. Nos. 2,881,052, 4,075,308 and 4,251,224.) Although the resulting aqueous solutions are quite dilute (containing e.g. about 5 to 10 grams of ClO.sub.2 per liter), they still represent a more compact and convenient form of ClO.sub.2, which can be handled with reasonable safety and economy.